Technical Field
The present disclosure relates to a thin-film-transistor (TFT) backplane for a display device, and more particularly, to a TFT backplane for more power efficient operation of the display and a method of fabricating such a TFT backplane.
Description of the Related Art
Flat panel displays (FPDs) are employed in various electronic devices such as mobile phones, tablets, notebook computers as well as televisions and monitors. Examples of the FPD includes a liquid crystal display (LCD), a plasma display panel (PDP), an organic light emitting diode (OLED) display as well as an electrophoretic display (EPD).
Generally, pixels in a FPD are arranged in a matrix form, and generate light (luminescence) upon electrical activation from an array of thin-film-transistors (TFTs), also known as TFT backplane. A TFT backplane plays an important role in a FPD as it functions as a series of switches to control the current flowing to each individual pixel. TFT backplanes that have been developed until recently can be categorized into two primary types, one using TFTs with amorphous silicon (a-Si) active layer and the other using TFTs with polycrystalline silicon (poly-Si) active layer.
A TFT with a-Si active layer generally has lower carrier mobility (μ) than that of a TFT with poly-Si active layer. Thus, making a high speed drive circuit (e.g., pixel circuit, gate drive integrated circuit, data drive integrated circuit) for a display is difficult with the TFT backplane employing a-Si TFTs.
In case of heat-treating a layer of amorphous silicon by using a laser beam, the layer of amorphous silicon can become a polycrystalline silicon active layer. The material according to this process is generally referred to as low-temperature polycrystalline silicon (LIPS). In general, the carrier mobility (μ) of LIPS TFTs is higher than the a-Si TFTs by as much as 100 times (>100 cm2/V·s). Despite significantly high carrier mobility of LIPS TFT, there is a downside as LIPS TFTs of a backplane has variations in their threshold voltages (Vth) due to a grain boundary. Such non-uniform threshold voltages among the TFTs employed in a TFT backplane may result in display non-uniformity referred to as the “mura.” For this reason, a display drive circuit implemented with LIPS TFTs often requires a complex compensation circuit, which in turn, occurs a problem as it increases the manufacturing time and cost of the display.
For flexible displays, a-Si TFTs or LIPS TFTs of the backplane need to be formed at low temperature to prevent thin plastic or glass substrates from degrading. However, because lowering the temperature during the fabrication process degrades the performance of the TFTs, there was a limitation in applying the above TFTs to flexible displays.
Due to the above problems of the silicon based TFTs, other type of backplane technology, which employees TFTs having an active layer formed of a metal oxide material has been suggested. In particular, oxide TFTs are regarded as an alternative to silicon based TFTs because of their high mobility (>10 cm2/V·s) and low process temperature (<250° C.), compared to those of a-Si TFTs. The lower leakage current and the scalability to any glass size make the oxide TFT a promising candidate for making a high performance TFT backplane for displays at low cost.
Stable and high-yield production of a TFT backplane employing oxide TFTs requires optimization of the TFT design, dielectric and passivation materials, oxide film deposition uniformity, annealing conditions, and more. Also, manufacturing process variations need to be minimized in order to tightly control the operating characteristics of such TFTs, including their threshold voltages. For example, adopting the etch-stopper type oxide TFTs can improve the reliability, but such a design suffers from high parasitic capacitance and complicates the manufacturing process. Further, the etch-stopper type limits how short the TFT channel can be, thereby affecting the overall size of the display backplane or the aperture ratio in the pixels of the display. As such, there are difficulties for designing the driving circuitry for a display.